The present invention relates generally to batteries. More particularly, the present invention relates to alkali metal batteries, i.e., batteries which have anodes which include as active material lithium, potassium, sodium, or another alkali metal of group IA of the Periodic Table or an alloy thereof.
Implantable cardiac defibrillators are used to prevent sudden death from lethal arrythmia. In operation, the defibrillator device continuously monitors the heart rate of the patient, is able to recognize ventricular fibrillation, and subsequently delivers high energy shocks to defibrillate the heart. Defibrillators are expected to function alone or in conjunction with a pacemaker. Thus, the power source must be of high capacity, low self-discharge, have good pulsing behavior, and be reliable. In addition, the implantable battery must deliver a current pulse and rapidly recover its open circuit voltage. In order to provide these operating characteristics, a lithium battery is considered to be a good power source.
The cathode of an alkali metal battery or cell is of a solid type and may have as active material thereof a suitable material such as, for example, carbon fluoride, a metal oxide, or a metal oxide bronze such as silver vanadium oxide, as disclosed in U.S. Pat. No. 4,830,940 to Keister et al, which patent is assigned to the assignee of the present invention and which patent is incorporated herein by reference. A xe2x80x9ccellxe2x80x9d is meant to refer to a battery or to a compartment thereof containing electrodes and electrolyte for generating electric current. The anode of such a battery or cell may be constructed in a serpentine fashion with cathode plates inserted between each of the convolutions thereof on both sides thereof. The electrolyte for an alkali metal battery or cell may be of the liquid organic type which comprises a suitable lithium salt and a suitable organic solvent. Both the anode and the cathode plates are usually incapsulated in an electrically insulative separator material.
Various kinds of separator material have been used for providing electrical insulation between the anode and cathode of various batteries. Examples of such separators are found in U.S. Pat. Nos. 4,741,979; 4,731,304; 4,650,730; 4,444,854; 4,288,503; 4,273,840; and 4,201,838. A separator commonly used in lithium batteries is a polypropylene laminate consisting of a non-woven layer of polypropylene and a membrane of polypropylene.
Safety problems may result due to external electrical or mechanical abuse or internal failures of lithium batteries. An internal or external short circuit causes the cell temperature to rise. If the internal cell temperature of a lithium battery reaches the lithium melting point of about 180xc2x0 C., the melting of the lithium can result in rapid exothermic reactions that may cause catastrophic rupture of the cell. An approach to improving the abuse resistance of the cell so that such an event does not occur is by lowering the cell temperature during abuse conditions by preventing current flow, i.e., by providing what might be termed a xe2x80x9cthermal fusexe2x80x9d. In a thermal fuse, separator material melts to close up the pores therein or melts onto one or both of the electrodes so as to block the passage of ions in the solution between the electrodes and thereby shut down or partially shut down the battery after which it may begin to cool down or at least not increase further in temperature. If such cool down begins before the melting point of 180xc2x0 C. of lithium is reached, catatrophic rupture of the cell may be avoided. In providing such a thermal fuse, it is therefore considered desirable to provide a greater margin of safety than is provided by the above-discussed polypropylene separator wherein the temperature must rise to a point in excess of about 150xc2x0 C., the melting point of polypropylene, before thermal shut-down can even begin to occur.
U.S. Pat. Nos. 4,650,730 and 4,731,304 to Lunquist et al disclose a battery separator in the form of a multiply microporous sheet product wherein one ply is a sheet capable of transforming to a substantially non-porous membrane at a temperature between 80xc2x0 C. and 150xc2x0 C. while essentially maintaining its length and breadth dimensions and a second ply is a sheet capable of maintaining its dimensions and porosity at temperatures from ambient to at least about 10xc2x0 C. greater than the transformation temperature of the first sheet for a purpose of preventing the occurence of overheating and thermal runaway in lithium batteries. The two sheets are bonded together to provide a unitary product. The polymeric compositions for each of the sheets is disclosed as being from classes of polymers capable of forming a microporous sheet such as polyolefins, polysulfones, polyvinyl chloride, polyvinyl fluoride, polycarbonates, and the like. As further disclosed therein, the polyolefin can be selected from homopolymers such as polyethylene or polypropylene or from various other polyolefins. Films disadvantageously tend to split along the machine direction to compromise the structural integrity of the separator and thus perhaps worsen a short circuit condition.
U.S. Pat. No. 4,741,979 to Faust et al discloses a composite separator for providing thermal protection in lithium batteries which comprises a thermal fuse and a film bearing the thermal fuse. The thermal fuse is disclosed as being prepared from a non-woven polyethylene coated polyester fabric which is coated with a wax material. It is indicated that the wax is intended to melt to form the thermal fuse. This layer of waxed fibers is disclosed as being pressure laminated to a microporous polypropylene film. It is disclosed that a wide varity of woven and non-woven fabrics including a polyethylene blend can be used as the film layer. Another separator assembly disclosed in Faust et al is a prelaminate of a non-woven polypropylene fabric and a microporous polypropylene onto which the wax is sprayed. The provision of such wax coatings to provide the thermal fuse do not lend to ease of manufacture of the separator.
It is considered desirable to provide an improved thermal protection separator for lithium and other alkali metal batteries. Among the attributes which such an improved separator desirably has is that it provide thermal shut down at a low temperature so that a large margin of safety against cell rupture can be achieved, that this large margin of safety be achieved without sacrifice of battery performance, and that the separator have good strength and tear and puncture resistance, so that the physical integrity of the separator may be maintained during short circuit conditions so that no gaps develop which may worsen the conditions. It is also desirable that the separator materials be such as to lend to ease of separator manufacture for reduced cost.
It is accordingly an object of the present invention to provide a thermal protection separator for a lithium battery as well as other alkali metal batteries which is reliable, easy to manufacture, provides an increased margin of safety without significantly sacrificing performance, and has good strength, tear resistance, and puncture resistance for maintaining its physical integrity for improved reliability.
In order to provide such an improved thermal protection separator for an alkali metal battery or cell, in accordance with one form of the present invention the thermal protection separator for an alkali metal battery or cell comprises a layer of microporous polyethylene film or other suitable microporous film having a melting point which is less than about 135xc2x0 C. and a layer of non-woven polypropylene or other suitable non-woven material having a melting point which is at least about 10xc2x0 C. higher than the melting point of the microporous film, wherein the microporous film provides a low melting point so that the margin of safety can be increased without significantly sacrificing performance and wherein improved reliability may be achieved by the microporous film providing good puncture resistance, and the non-woven mesh may provide good strength and tear resistance.
In another form of the present invention, the thermal protection separator comprises the combination of polyethylene on one of the cell electrodes and polypropylene on the other of the cell electrodes. The combination can be in various forms, for example both components can be film, i.e. microporous, or both can be nonwoven, one component can be film and the other non-woven, or one component can be film and the other a laminate film and non-woven. in addition to enhanced cell safety, advantages include a lower short circuit peak temperature on the cell or battery casing and avoidance of rupture on short circuit.
U.S. Pat. No. 4,273,840 to Machi et al, which is hereby incorporated herein by reference, discloses in example 2 thereof one type of separator which comprises a composite sheet consisting of non-woven cloth composed of polypropylene fibers and a polyethylene film for alkaline batteries, particularly zinc-silver oxide batteries using silver oxide or peroxide as cathode and zinc as anode, and nickel-zinc secondary batteries or other batteries using zinc as negative active material. Acrylic acid is grafted to the laminate sheet. Machi et al states that such a separator has a longer cycle-life than a cell using a separator composed of regenerated cellulose membrane and that cells using the separator required a smaller anode-to-cathode spacing which contributed to improved cell performance. Machi et al does not provide any teaching of suggestion relative to an improved thermal protection separator for a lithium or other alkali metal battery since the melting of the materials of the anode and cathode thereof does not result in the exothermic reactions that may cause rupture of a cell when lithium or another alkali metal melts. Thus, a thermal fuse would not be considered necessary in the battery of Machi et al, and one would therefore not consider Machi et al to be useful for providing any teaching relative to thermal fuses for lithium or other alkali metal batteries.
The above and other objects, features, and advantages of the present invention will be apparent in the following detailed description of the preferred embodiments thereof when read in conjunction with the accompanying drawings wherein like reference numerals denote like parts in the various views.